Titanium dioxide is a widely-used but increasingly expensive pigment in paints and other coating compositions. In many paint formulations titanium dioxide represents the single most expensive raw material. A number of techniques and ingredients have been suggested for reducing the amount of titanium dioxide in coating composition formulations while still providing acceptable light scattering or opacity. One such ingredient is EVOQUE™ Pre-Composite Polymer supplied by Dow Chemical Company. EVOQUE products are presently available in four grades (EVOQUE 1140, EVOQUE 1180, EXP-4340 and EXP-4463) that are said to have differing degrees of reactivity towards titanium dioxide. According to its supplier, EVOQUE pre-composite polymer “combines with TiO2 to form a polymer-pigment composite which improves both the wet and dry hiding efficiency of the pigment.”
Also according to its supplier, “A minimum level of EVOQUE Pre-Composite Polymer is required to fully cover the surface of TiO2 in order to improve hiding efficiency and allow for developing stable formulations.” The recited minimum level is sometimes called the “saturation level” or “saturation level of adsorption” (defined below). The saturation level will depend on the pigment volume concentration (PVC, also defined below), with lesser polymer addition levels being needed to reach the saturation level of adsorption at low PVC and greater polymer addition levels being needed to reach the saturation level of adsorption at high PVC.
Some patents describing composite pigment particle-forming polymers that are believed to be like the polymer present in EVOQUE Pre-Composite Polymer say that other copolymers may be present during formation of the composite particle, so long as such other copolymers do not inhibit or substantially interfere with adsorption of the polymer particle to the pigment particle.
The currently recommended method for making latex paints using EVOQUE Pre-Composite Polymer is carried out by dispersing titanium dioxide into a mixture of EVOQUE Pre-Composite Polymer, water and defoamer to form a composite, and then combining the composite with the remaining paint ingredients including an acrylic latex paint binder in order to form a finished paint formulation. In this method, the acrylic latex paint binder is not combined with the composite until after the composite has been formed. The resulting paint or other coating composition may attain a given degree of pigment hiding at a somewhat reduced titanium dioxide pigment level, or may attain a somewhat increased degree of pigment hiding at a given titanium dioxide pigment level. However, the potential savings in titanium dioxide raw material costs and the potential benefits in pigment hiding efficiency are offset by the EVOQUE product cost.
From the foregoing, it will be appreciated that what remains needed in the art are improved titanium dioxide-containing coating compositions having even lower raw material cost, even greater pigment hiding efficiency, or both lower cost and greater efficiency. Methods for the manufacture of such compositions are disclosed and claimed herein.